Various ways exist today to access IP services over the Internet. Today's cellular networks for instance allow users to establish a connection while they are roaming in different geographical areas. Third generation (3G) networks (UMTS, CDMA2000 networks . . . ) for example are now deployed in most of industrialised countries, allowing simultaneously wide-area wireless voice telephone calls and wireless data transfer all in a mobile environment.
A cellular network is a radio network made up of a number of juxtaposed cells, each of which being served by at least one fixed-location transceiver known as cell site or base station. When joined together, the base stations provide radio coverage over a wide geographic area. This enables a large number of portable transceivers (user equipment such as mobile phones, computer, pagers, etc) to communicate with each other and with fixed transceivers anywhere in the network, via BSs, even if some of the transceivers are moving through more than one cell during transmission.
The cells constituting a cellular network may be of different ranges depending on the circumstances. Macrocells for instance correspond to cells providing the widest coverage area. Macrocells are usually served by high power cellular base station mounted on high structures (rooftops etc.). A cellular network may also include microcells, i.e. cells served by low power cellular base stations and providing smaller coverage areas. A cellular network comprising microcells is sometimes named microcell cellular network.
Cells known as femtocells have also experienced significant developments over the last years. A femtocell is an even smaller cellular cell served by a small cellular base station. Femtocell networks are typically designed for the realization of high density wireless deployments in urban areas (e.g. 3GPP cellular networks, WiMAX networks, etc.). The femtocell concept allows users to install their own small base station in indoor environment using licensed spectrum, typically for domestic use or for the purpose of small businesses. In other words, a femtocell enables service providers to extend service coverage indoors, especially where access would otherwise be limited or unavailable with conventional macrocell networks.
Based on access control, the 3rd Generation Partnership Project (3GPP) has classified femtocell base stations into two categories: the Open Access femtocell base stations and the Closed Access femtocell base stations. An Open Access femtocell base station can serve any user equipment (UE) whereas a Closed Access femtocell base station only serves UEs which are members of a particular group called CSG. Such femtocells providing restricted access are known as CSG femtocells. The owner of a femtocell base station may for instance decide to restrict access to his own femtocell by creating a CSG, on the ground that he is the one assuming the expense of maintenance and the broadband connection to his premises.
FIG. 1 depicts an example of a terminal accessing IP services via a CSG femtocell network.
In this document, the term “terminal” will designate any UE such as a mobile phone, a PDA, a computer etc. operable to access a wireless access network.
In addition, it will be assumed in this example that the user of terminal 102 has subscribed to have authorised access to the CSG femtocell 106. The terminal may thus access IP services on the Internet 110 via CSG femtocell 106 which serves as a relay between terminal 102 and the core network 108. Once terminal 102 is attached to the base station 104 of CSG femtocell network 106, the core network 108 can route IP packets between the CSG femtocell network 106 and the Internet 110.
According to the existing 3GPP standard (TS 25.367 v9.2.0, “Mobility Procedures for Home NodeB; Stage 2; Overall Description (Release 9), December 2010”), each CSG femtocell is associated with:                a specific carrier frequency identified by a corresponding frequency identifier (e.g. UTRA Absolute Radio Frequency Channel Number—UARFCN in UMTS system . . . ),        a specific Primary Scrambling Code (PSC), and        a unique numeric identifier called CSG Identity (CSG ID).        
Carrier frequencies and PSCs of CSG femtocells are dependent upon the operator's deployment strategy, network planning, frequency allocation, etc. In most cases, a given network operator implements the same carrier frequencies and PSCs for each deployed CSG femtocell throughout a country.
The specific characteristics and use of each of the above parameters will be further described below.
To connect to a CSG femtocell network, terminal 102 must perform a discovering procedure followed by a selecting procedure (collectively named as discovering and selection procedure).
The discovering procedure implemented by terminal 102 consists of scanning all frequencies in the supported frequency bands to discover reachable CSG femtocell networks. It should be noted that, in this document, a “reachable” network (or cell) designates a network (or cell) that a terminal may physically reach because the two elements are located sufficiently closed to each other. However, access to a reachable network (or cell) may or may not be authorised depending in particular upon subscription rights of the terminal's user (i.e. user subscription data).
During this scanning step, terminal 102 executes a channel synchronisation mechanism so as to synchronise with CSG femtocell frequencies (i.e. frequencies used by reachable CSG femtocells) and to discover the respective PSC associated with each discovered reachable CGS femtocell.
Terminal then deciphers a data stream broadcasted by each of the discovered reachable CSG femtocell, using the obtained PSCs as decoding keys. When the discovered CSG femtocells are of the 3GPP type, the data streams in question correspond to cell- or system-specific data broadcasted by the discovered CSG femtocell networks.
As each CSG femtocell is associated with a specific PSC, terminal 102 is able to distinguish between reachable CSG femtocells by recognising their respective PSC.
This decoding (or deciphering) step allows terminal 102 to determine the unique CSG ID broadcasted by each of the discovered CSG femtocell.
Once the CSG ID of each discovered CSG femtocell is determined, terminal 102 determines which among the discovered CSG femtocells terminal 102 is authorised to access. To do so, terminal 102 has a list (usually named “whitelist”) which may contain one or a plurality of authorised CSG IDs. Each authorised CSG ID corresponds to an authorised CSG femtocell, that is, a CSG femtocell that terminal 102 is authorised to access based on its user subscription data. It should however be noted that this whitelist may be empty. This is for instance the case when a new terminal is put into service without any preconfigured CSG cell information stored therein.
If the whitelist of terminal 102 does contain at least one authorised CSG ID, terminal 102 compares the CSG IDs broadcasted by the discovered CSG femtocells with the preconfigured authorised CSG IDs defined in the whitelist.
Terminal 102 then selects a discovered reachable CSG femtocell recognised as an authorised CSG femtocell during the discovering mechanism (selection procedure). The selecting procedure may be either manual or automatic. According to the manual selection mechanism, the user of terminal 102 manually selects the CSG ID of an authorised CSG femtocell he wishes to connect to. On the other hand, the automatic selection mechanism allows terminal 102 to automatically select an authorised CSG femtocell. In this respect, it is worth noting that if the whitelist of terminal 102 does not contain any CSG ID, terminal 102 must solely depends on the manual selection mechanism.
In the particular example of FIG. 1, terminal 102 discovers CSG femtocell 106 and determines that it has authorised access to this particular CSG femtocell based on the discovering procedure described above. Therefore, terminal 102 selects CSG femtocell 106 and attaches to the corresponding bases station 104 to access IP services on the Internet 110 via CSG femtocell 106.
However, the existing CSG femtocell discovering and selection procedure specified by 3GPP group presents major drawbacks. For instance, a terminal is compelled to perform a scanning search throughout the entire spectrum of the supported frequency bands to complete the discovering procedure. Such a scanning search demands significant amount of battery power, thereby resulting in unnecessary wastage of UE's battery power. Furthermore, this search is particularly time consuming and therefore may delay the CSG femtocell attachment process, especially in an area densely populated by a number of femtocell base stations.
Further, the execution of this complete scanning search does not guarantee the discovering of all accessible CSG femtocell networks. The distribution of authorised and unauthorised (i.e. forbidden) CSG femtocells for a particular terminal can vary on a regular basis depending on the operator's deployment strategy. Therefore, a terminal may discover a CSG femtocell network and unnecessarily attempt to connect thereto even though this particular network is no longer authorised for this terminal. Conversely, a terminal may ignore a discovered CSG femtocell network while this network has been recently made available for this terminal.
There are however some optional features proposed by 3GPP to optimize the discovering and selection procedure of CSG femtocells, where the femtocells can also broadcast carrier frequency indication, PSC range in use, etc. Since, whether a specific femtocell will broadcast this optional information or not depends on whether the equipment manufacturer has implemented this feature on the femtocell, there is no guarantee that the UE will be able to use the feature.
Accordingly, there is a need for an improved mechanism which alleviates the drawbacks of the existing CSG femtocell discovering and selection procedure.